The possibility of producing amplified coherent electromagnetic radiation, by collinear passage of the radiation and of a relativistic electron beam through a sequence of electric or magnetic fields of alternating polarity, has been recognized since the first publication by H. Motz, Journal of Applied Physics 22 527 (1951) on the subject. Motz considered a sequence of alternating direction magnetic fields, regularly spaced and transversely oriented relative to the common direction of travel of light beam and electron beam, Let L.sub.o be the fundamental period of variation of direction of the sequence of transverse magnetic fields and let the beam electrons move with velocity v.congruent.c. Photons will be emitted by the electrons, at a frequency .nu. which will depend upon angle of observation .theta. relative to the common beam direction according to .nu.=(V/L.sub.o)(1-cos .theta.). For a highly relativistic electron beam and modest transverse magnetic field stengths, most of the radiation appears in the forward direction, in a narrow cone of half angle of the order of ##EQU1## is the electron total energy. This approach requires a bunched electron beam with sinusoidally varying orbit, and the Motz analysis is essentially classical and relativistic.
Motz, Thon and Whitehurst, in Jour. of Appl. Phys., 24 826 (1953), further considered the co-propagating light beam and electron beam in a waveguide, obtained some interesting general classical relativistic relationships for electron orbits in a spatially varying B-field, and reported the experimental observation of visible and millimeter wavelength radiation for field strengths B.perspectiveto.3,900 and 5,600 Gauss.
K. Landecker, in Physical Review 86 852 (1951) proposes up-conversion of the frequency of a substantially monochromatic electromagnetic wave by reflection of the wave from a parallel relativistic electron beam moving in the opposite direction, somewhat analogous to reflection of a beam of light by a mirror moving at relativistic velocity.
In Proceedings of the Symposium on Millimeter Waves (Polytechnic Press, Brooklyn 1960) p. 155, Motz and Nakamura analyzed the amplification of a millimeter wavelength em. wave interacting with a relativistic electron beam in a rectangular waveguide and a spatially oscillatory magnetic field, using a model of J. R. Pierce. The analysis was purely classical, and the gain was rather modest.
Pantell, Soncini and Puthoff discuss some initial considerations on stimulated photon-electron scattering in I.E.E.E. Journal of Quantum Electronics QE-4 905 (1968). Collinear scattering, with the incident photon energy h being &lt;&lt; incident electron energy E.sub.e1 and periodic deflection of the electron beam by a microwave radiation field, is analyzed briefly; and a Compton scattering laser is proposed, using the input/output wavelength relation .nu..sub.out =4(E.sub.e1 /m.sub.e c.sup.2).sup.2 .nu..sub.in. Useful gain from the device appears to be limited to the middle-high infrared frange .lambda.&gt;20 .mu.m.
Mourier, in U.S. Pat. No. 3,879,679, discloses a Compton effect laser that proceeds from the same principles as Pantell et al, supra. This invention, like that of Pantell et al, appears to require provision of an electron storage ring or the like for rapidly moving electrons and an optical cavity that is a part of the ring, for causing electron-photon scattering.
R. M. Phillips, in I.R.E. Transactions on Electron Devices, 231 (October 1960), used a periodic magnetic field, whose period may vary, to focus and axially bunch an electron beam traveling in an unloaded waveguide, together with a monochromatic light beam, to increase light beam energy at the expense of electron beam kinetic energy. The electron beam velocity was adjusted so that a beam electron travels one period L along its trajectory in the time required for the light beam (of wavelength .lambda.) to travel a distance L+.lambda.. The electron then senses only the retarding porition or only the accelerating portion of the electromagnetic wave. This approach converts transverse momentum, arising from the presence of the electromagnetic wave, into changes in axial momentum of the electron beam so that beam bunching occurs.
J. M. J. Madey, in Journal of Applied Physics 42 1906 (1971), discusses stimulated emission of bremsstrahlung by a relativistic electron into a single electromagnetic mode of a parallel light beam, where both electron and light beam move through a periodic, transverse d.c. magnetic field. Quantum mechanical and semi-classical calculations of transition rates and gain indicate that finite, practical gain is available in the infrared and visible portions of the optical spectrum. These considerations are incorporated in U.S. Pat. No. 3,822,410, issued to Madey for tunable apparatus for generation/amplification of coherent radiation in a single or a few closely spaced electromagnetic modes.
Elias, Fairbank, Madey, Schwettman and Smith, in Physical Review Letters 36 717 (1976), have reported experimental gain of 7% per pass length of 5.2M, using an optical beam of wavelength .lambda.=10.6 .mu.m from a CO.sub.2 laser interacting with a relativistic electron beam (E.sub.e .apprxeq.24 MeV) having an associated peak current of 70 milliamps in the presence of a periodic magnetic field of wavelength 3.2 cm and strength B=2.4 kiloGauss, using a conventional free electron laser approach.
Deacon, Elias, Madey, Ramian, Schwettman and Smith, in Physical Review Letters 38 892 (1977), returned to the same configuration used by Elias et al above, using an optical beam of wavelength .lambda.=3.417 .mu.m, a relativistic electron beam of energy E.sub.e +43.5 MeV and an optical cavity length of 12.7M, and report the first operation of a free electron laser oscillator, operated above threshold. Gain is modest and depends upon system length.
McDermott, Marshall, Schlesinger, Parker and Granatstein report on a different experimental approach to free electron laser operation in Physical Review Letters 41 1368 (1978), using stimulated Raman backscattering in the presence of a periodic magnetic field (.lambda..sub.w .apprxeq.0.8 cm). Using a 40 nsec electron beam pulse length from an accelerator and an optical cavity of length L=150 cm, four round trip passes of the electromagnetic radiation are made for a single e-beam pulse, thus approaching conventional laser performance. Coherence of the laser signal is improved by an order of magnitude over the estimated single pass, super-radiant value of .DELTA..lambda..sub.s /.lambda..sub.s =10-20%; and the laser output power (0.5-1.0 Megawatt) and (high) gain are no longer strongly dependent upon system length. The wiggler had a length of 40 cm and a strength B.apprxeq.400 Gauss.
Hirshfield, in U.S. Pat. No. 3,398,376 for a relativistic electron cyclotron maser, discloses and claims use of an axial, monoenergetic relativistic electron beam (E.sub.kinetic .apprxeq.5 keV) a spatially-varying longitudinal magnetic field coaxial with the beam, a weaker, transverse periodic electric or magnetic field with a resulting helical pitch matching that of the electron motion at the predetermined beam velocity and a cavity resonator with a mode frequency matching that of the cyclotron frequency of the resulting spiraling electrons. The apparatus relies upon electron cyclotron radiation.
A combination free electron laser/gas laser with high pulse repetition rates is taught by U.S. Pat. No. 4,189,686, issued to Brau, Rockwood and Stern. In the embodiment disclosed, the free electron laser operates at ultraviolet wavelengths. The monoenergetic electron beam is initially bunched and accelerated to .apprxeq.10 MeV kinetic energy and directed into and out of a multiplicity of serially arranged free electron lasers by turning magnets positioned at the ends of these lasers; finally, the electron beam is directed axially through a gas laser to utilize and convert additional electron beam energy to electromagnetic energy. The free electron laser appears to be of conventional form, utilizing fixed period magnetic fields to produce electron bremsstrahlung radiation and an optical resonator for light beam amplification.